Scientists reported these findings Sunday at the 2004 AAAS Annual Meeting. The latest information about carbon dioxide fertilization � by which plants soak up carbon from the atmosphere � "really paints a different picture of the way the world works," said panelist Chris Field of the Carnegie Institution of Washington.

In a book edited by Field and scheduled for publication in late February, researchers concluded that the land contains many large pools of carbon that are likely to shrink in the coming century.

A key reason for the differing conclusions, Field and his colleagues found, is that the predictions of the Intergovernmental Panel on Climate Change and other studies have relied on models that don't reflect some of the major processes by which carbon circulates through the environment.

Field and his colleagues also have discovered in a previous study that there may not be enough biologically available nitrogen to support certain optimistic estimates of the land's capacity for carbon fertilization.

"If you put together these two lines of evidence, we're looking at a future in which we may see less carbon being removed from the atmosphere," Field said.

"The fact that carbon dioxide fertilization is likely to be more modest does not imply that carbon management through planting trees is a bad idea," Field explained. "Planting trees is a great idea. It's just that the trees will grow at their 'normal' rates or slightly faster, rather than at supercharged rates."

Field co-organized the symposium with Stephen Schneider of Stanford University, pulling together speakers studying a variety of different landscapes, who met at the Annual Meeting of the American Association for the Advancement of Science (AAAS).

Jeff Dukes of the University of Massachusetts Boston has been monitoring changes in a California grassland, over five years of exposure to various types of environmental change. Presenting a new analysis covering five years of data, Dukes reported that their response to elevated atmospheric carbon dioxide was minimal.

"Carbon dioxide may boost or suppress grassland productivity in some years, but over the longer term it's pretty much a wash," Dukes said.

A seven-year study of a pine forest has produced similar results. According to William Schlesinger of the Nicholas School at Duke University, the Duke Forest Free Air Carbon Enrichment (FACE) experiment showed that enriching carbon dioxide in a young loblolly pine initially enhanced growth by 10 to 20 percent, with higher values in the driest years. But, various lines of evidence suggest that soil nutrients are deficient to support long-term growth stimulation, Schlesinger said.

Schlesinger stressed that planting trees is an effective way to sequester carbon, but "shouldn't expect those trees to grow much faster in the high CO2 world of the future."

Ultimately, the Earth's ability to take up carbon will depend on the oceans. The oceans have already absorbed some 400 billion tons of fossil fuel carbon dioxide, and this trend will continue; ocean uptake now is more than 20 million tons of carbon dioxide per day, according to Peter Brewer of the Monterey Bay Aquarium Research Institute.

"But is this a blessing or a problem?" He asked.

Some researchers have considered direct ocean disposal of carbon dioxide, raising questions as to the impact of changing ocean acidity, or "pH" on marine life. Experiments to examine the impact of elevated carbon dioxide levels on the land are commonplace.

Brewer reported on the first small-scale ocean experiments, in which his research team added carbon dioxide to the deep-sea off California, and thus perturbed the pH of the surrounding ocean, exposing animals to waters that may simulate the ocean of the late 21st century.

He described some new experimental techniques that should make it possible to extend these types of experiments, making them both spatially larger and longer-lasting.

"It's the only way to find out how coral reefs, deep-sea fisheries and other marine environments will react to a change in ocean pH; you have to do the experiment," he said.

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